This invention relates to an engine idling speed control apparatus for controlling the amount of air permitted to enter the engine so as to maintain the engine speed at a target value when the engine is idling.
For example, Japanese Utility Model Kokai No. 1-179148 discloses an engine idling speed control apparatus which includes an auxiliary air control valve provided in an auxiliary air passage bypassing a throttle valve situated within an engine induction passage. The engine idling speed control apparatus is arranged to change the duty factor of an electrical pulse signal applied to operate the auxiliary air control valve when the engine is idling. The duty factor change is made in a manner to provide a feedback control correcting the air flow through the auxiliary air passage to maintain the engine idling speed at a target value. The duty factor ISC.sub.ON is calculated as ISC.sub.ON =ISC.sub.TW +ISC.sub.CL where ISCT.sub.TW is a basic control factor calculated as a function of engine coolant temperature TW and ISC.sub.CL is a feedback correction factor containing integral plus proportional terms generated in response to the sensed deviation of the actual engine speed N.sub.e from the target value N.sub.SET. For example, when an external load is produced to decrease the actual engine speed N.sub.e, it is required to increase the duty factor ISC.sub.ON so as to zero the deviation of the actual engine speed N.sub.e from the target engine idling speed value N.sub.SET. Since the conventional engine idling speed control apparatus is arranged to increase the duty factor gradually while monitoring the engine speed change, however, it requires much time to zero the deviation and has a slow response.